1. Field of the Invention
The present invention relates to a radiation image conversion panel that is employed in a radiation image converting method which takes advantage of phosphor, and a manufacturing method therefor.
2. Description of the Related Art
Radiography, which employs a combination of a silver halide photosensitive material (also referred to simply as a sensitive material) and a radiation intensifying screen, is known as a method of obtaining a radiation image for medical diagnosis and radiation images of various objects in a nondestructive manner. These images are employed for diagnosis, injury inspection, etc. This radiography is the process of forming a radiation image in the sensitive material, by irradiating radiation, transmitted through or emitted from a subject, to phosphor in the radiation intensifying screen to excite the phosphor and converting the radiation to near ultraviolet light or visible light. The formed radiation image is diagnosed and inspected. With one or both surfaces of the screen in intimate contact with the sensitive material that has one or two silver halide emulsifier layers at one or both surfaces of a support body, radiation is irradiated via a subject and an image is developed, whereby the radiation image is formed.
As a method that replaces this radiography, there is a radiation image converting method such as that described in Japanese Unexamined Patent Publication No. 55(1980)-12145. The method employs stimulatable phosphor, which absorbs radiation energy and, when excited with electromagnetic waves (such as visible light, infrared rays, etc.), emits the stored radiation energy in the form of fluorescence. This method utilizes a radiation image conversion panel (also called a storable phosphor sheet) containing stimulatable phosphor. Radiation, transmitted through or emitted from a subject, is irradiated to the stimulatable phosphor in this panel. When excited in a time series manner with electromagnetic waves (excitation light) such as visible light, infrared rays, etc., the stimulatable phosphor emits the stored radiation energy as fluorescence (photostimulated luminescent light). This fluorescence is read photoelectrically and converted into an electrical signal. Based on the electrical signal obtained, the radiation image of the subject is reproduced as a visible image.
The aforementioned radiation image converting method has the advantage that a radiation image with abundant information can be obtained with considerably less exposure, compared with the conventional radiography which employs a combination of radiography film and an intensifying screen. Therefore, this method has great utilization value in performing direct medical radiation photographing at the time of X-ray photographing for medical diagnosis.
The radiation image conversion panel that is employed in the radiation image converting method consists basically of a support body and stimulatable phosphor layer provided on one surface of the support body. Note that in the case where the phosphor layer is a self-supporting type, the support body is not always required. In addition, a transparent protection film is generally provided on a surface of the stimulatable phosphor layer remote from the support body (i.e., a surface on the side of the stimulatable phosphor layer not facing the support body). The protection film is used for protecting the phosphor layer from chemical degeneration and physical shock.
The stimulatable phosphor layer is composed generally of stimulatable phosphor particles and a binding agent for containing and supporting them in a dispersed state. The stimulatable phosphor particles have the property of absorbing radiation such as X-rays and, when irradiated with excitation light, exhibiting photostimulated luminescence. Therefore, the radiation transmitted through or emitted from a subject is absorbed in proportion to the amount of radiation in the stimulatable phosphor layer of the radiation image conversion panel, so that a radiation image for the subject is formed in the panel as a stored image of radiation energy. This stored image can be emitted as photostimulated luminescent light by irradiating the aforementioned excitation light. Therefore, it becomes possible to reproduce the stored image of radiation energy by photoelectrically reading the photostimulated luminescent light and converting it into an electrical signal.
While the radiation image converting method is a very advantageous image-forming method, as described above, the radiation image conversion panel employed in this method is also desired to be highly sensitive and to provide satisfactory picture quality (e.g., sharpness, graininess, etc.), as with the intensifying screen employed in the conventional radiography.
The sensitivity of the radiation image conversion panel is basically dependent on the total of photostimulated luminescence emitted by the stimulatable phosphor particles contained in the panel. The total of photostimulated luminescence varies not only with the luminescent brightness of the phosphor particles themselves but also with the phosphor content of the phosphor layer. Because a greater phosphor content means that absorption for radiation such as X-rays is also greater, even higher sensitivity is obtained and, at the same time, picture quality (particularly, graininess) enhances. In the case where the phosphor content of the phosphor layer is constant, the thickness of the phosphor layer can be made thinner as the phosphor layer is densely filled with phosphor particles. Consequently, the spread of excitation light by scattering can be reduced and high sharpness can be obtained accordingly.
As one of the radiation image conversion panels having a phosphor layer densely filled with phosphor particles, the present applicant has already filed a radiation image conversion panel and a manufacturing method therefor, in which a void ratio for a phosphor layer has been reduced by performing a compression process on the phosphor layer (see Japanese Unexamined Patent Publication Nos. 59(1984)-126299 and 59(1984)-126300).
The aforementioned radiation image conversion panel makes the density of the phosphor particles in the phosphor layer higher than that of the radiation image conversion panel theretofore made, by performing a compression process on the phosphor layer. Consequently, this radiation image conversion panel has excellent sharpness. However, it has the disadvantage that sensitivity reduction will occur, because some of the phosphor particles are destroyed by the compression process. For this reason, investigations have been made in order to improve the dispersibility of stimulatable phosphor particles so that before the compression process, the void ratio of the phosphor particles is made as low as possible.
To improve the dispersed state of the stimulatable phosphor particles, there is a method of performing dispersion for a long period of time when preparing stimulatable paint. However, since great shearing force is exerted on the stimulatable phosphor particles when they are dispersed, there is a possibility that the characteristics of the stimulatable phosphor particles will be degraded. Furthermore, the method is undesirable for efficient operation, as it takes a long period of time to manufacture the radiation image conversion panel.
Hence, investigations have been made with respect to methods of effectively dispersing the aforementioned stimulatable phosphor particles without involving a great change to ordinary methods of manufacturing the radiation image conversion panel. As examples of these methods, there are a method of performing a surface process on stimulatable phosphor particles with a surface processing agent such as a silane coupling agent (see Japanese Patent Publication No. 6(1994)-31908) and a method of processing phosphor particles with a titanate coupling agent (see Japanese Patent Publication No. 8(1996)-540363). However, even these methods are insufficient as methods for obtaining a high-sensitive radiation image conversion panel by increasing the dispersibility and fill ratio of phosphor particles. This results from cases where the stability of the dispersion of the aforementioned stimulatable phosphor particles subjected to the surface process by use of the coupling agent is usually enhanced in the state of the coating solution, but compatibility for a resin component diminishes conversely, and is also because there are cases where the dispersed state of the final phosphor particles in the phosphor layer is not sufficiently improved.